/ Basics

Ohm's law in application

Now that I have my breadboard, I'm ready to make the easiest circuit of the world : power on a LED :D

I choose a blue LED, which allow an input voltage of 2.2V with a current of 18mA.

So I cannot plug my LED directly on Vcc (here 5V). I have to add a resistor to limit current and voltage in my LED.

Easiest circuit of the world

I think that you already know Ohm's law. I do too.

U = R * I

But for this simple circuit, do you know how to calculate R value ?

It's quite simple, but when I asked myself the question the first time, it was not clear to me that I had to choose the value of R to limit the current in the circuit.

So I want a current of 18mA, and 2.2V in my LED, and I've got a 5V input.

The resistor and the LED being connected in series, the resistor has to absorb the voltage that the LED cannot eat :

UR = UP1 - ULED = 5V - 2.2V = 3.8V

And to get 18mA in my circuit, I have to use Ohm's law to calculate R :

UR = R * I

R = UR / I = 3.8 / (18 * 10-3) = 211Ω

As I don't have such a resistor, I will use 220Ω.

Let's try it on our breadboard !
Circuit

To conclude, Ohm's law allows you to define the current and the voltage of your circuit.
In next posts, I think we will see other circuits a lot more complex. This basic rule will always be used. I have a lot to learn !